1. Technical Field
The description set forth herein relates generally to encapsulated materials having improved optical performance and a method of manufacturing thereof. More particularly, the description relates to utilizing a lubricant in encapsulated materials to improve optical performance.
2. Description of Related Art
Display technologies based on encapsulation of electrophoretic particles, multichromal rotatable elements and liquid crystals have many potential applications in fields such as digital document media, for example, electronic paper. High brightness and high contrast are two of the main performance requirements for digital document media applications.
Multichromal displays, also called twisting-ball displays, rotary ball displays, particle displays, bipolar particle light valves, etc., offer a technology for making a form of electric paper. Multichromal displays are addressable displays including a plurality of optically anisotropic balls, each of which can be selectively rotated to orient a desired surface for viewing by an observer. For example, multichromal displays can incorporate particles each having two distinct hemispheres, one black and the other white, with each hemisphere having a distinct electrical characteristic (e.g., zeta potential with respect to a dielectric fluid) so that the particles are electrically as well as optically anistropic. Other kinds of rotating elements are also known. For example, U.S. Pat. No. 4,261,653, which is incorporated herein by reference in its entirety, discloses a multilayer sphere, although it is made at least in part from glass and its use depends on an addressing scheme involving high-frequency electric fields.
U.S. Pat. No. 5,389,945, incorporated herein by reference in its entirety, shows that multichromal displays can be made that have many of the desirable qualities of paper, such as flexibility and stable retention of a displayed image in the absence of power, not found in conventional display media. Multichromal displays can also be made that are not paper-like, for example, in the form of rigid display screens for flat-panel displays. Multichromal displays are also disclosed in U.S. Pat. Nos. 5,262,098, 5,344,594, 5,717,514, 5,989,629, and 6,097,531, each incorporated herein by reference in its entirety.
In known bichromal displays, capsules or microcapsules include rotatable elements, such as black-and-white balls, commonly referred to as bichromal beads. Optionally, other bichromal balls may be used, as any combination of two colors may be used. These bichromal beads are embedded in a sheet of optically transparent material, such as an elastomer layer, which contains spheroidal cavities and is permeated by an oil such as a transparent dielectric fluid, such as a plasticizer. The fluid-filled cavities are sized to accommodate the bichromal beads, one bichromal bead per cavity, so as to prevent the rotatable elements from migrating within the sheet. The bichromal beads may be selectively rotated within their respective fluid-filled cavities, for example by the application of an electric stimulus, such as an electric field, so as to present either the black or the white hemisphere to observers viewing the surface of the sheet. Thus, by application of an electric field that is addressable in two dimensions (as by a matrix addressing scheme), the black and white sides of the bichromal beads can be caused to appear as the image elements (e.g., pixels or subpixels) of a displayed image.
Common bichromal capsule formulations may utilize X-5175 (X-5175 is available from Baker-Petrolite and is a poly(ethylene oxide-b-ethylene) block copolymer) to improve pigment charging and dispersion and thus improve optical performance. This formulation of the capsule is designated as “mainline” to distinguish it from various “custom” formulations, which exclude the X-5175. However, based on current encapsulation processes of mainline beads, the encapsulated rotatable elements exhibit low contrast properties. A primary reason for this low contrast may be attributed to the fact that under an electric field the rotatable element makes contact with the capsule wall before complete rotation, specifically approximately 180°, can occur. It is known in the art that rotatable element translation and rotation are related to the rotatable element monopole charge and dipole torque, respectively. Charge analysis further demonstrates that the X-5175 in mainline rotatable elements alters the balance between the monopole charge and the dipole charge. In the example of an encapsulated mainline bead, the dipole charge is less than the monopole charge causing the rotatable elements to come in contact with the wall before the rotatable element can complete a 180° rotation, typically only rotating about 90° to about 130°. One solution to address this problem is to utilize a higher viscosity fluid inside the capsule. The higher viscosity fluid slows down the translation motion of the rotatable element through viscous drag thereby providing time for the rotatable element to rotate prior to making contact with the capsule wall. However, the higher viscosity fluid also causes the rotation motion to become sluggish and thereby still hinders the rotatable element from fully rotating and thus allowing for any improvement in optical performance and contrast ratio properties.
Accordingly, there is a need for encapsulated materials and an encapsulation process thereof that allows a rotatable element located therein to rotate in a more desirable manner prior to making contact with a capsule wall. Such rotatable elements may achieve improved contrast ratio and optical performance.